The development of renewable transportation fuels is one of the key challenges of the twenty-first century. The current market is dominated by ethanol derived from yeast fermentation of sucrose and starch, and to a lesser extent by biodiesel (fatty acid esters) derived from triglycerides. Ethanol has limitations as a liquid fuel with a lower energy density relative to hydrocarbons. In addition, ethanol cannot be transported in conventional infrastructure due to its affinity for water and corrosive nature. Processes for the conversion of renewable carbon sources (biomass, sugars, oils) to hydrocarbon fuels offer an attractive alternative to bioethanol.
Isoprene (2-methyl-1,3-butadiene) is a key industrial chemical used primarily for the production of synthetic rubber. Currently isoprene is derived from petrochemical sources either directly by cracking of naphtha and other light petroleum fractions, or indirectly through chemical synthesis (See, for examples, H. Pommer and A. Nurrenbach, Industrial Synthesis of Terpene Compounds, Pure Appl. Chem., 1975, 43, 527-551; H. M. Weitz and E. Loser, Isoprene, in Ullmann's Encyclopedia of Industrial Chemistry, Seventh Edition, Electronic Release, Wiley-VCH Verlag GMBH, Weinheim, 2005; and H. M. Lybarger, Isoprene in Kirk-Othmer Encyclopedia of Chemical Technology, 4th ed., Wiley, New York (1995), 14, 934-952.) The resulting crude isoprene streams are typically subjected to extensive purification processes in order to remove numerous chemically similar impurities, many of which can interfere with subsequent transformation of isoprene to polymers and other chemicals.
In contrast, isoprene derived from biological sources contains very few hydrocarbon impurities and instead contains a number of oxygenated compounds such as ethanol, acetaldehyde and acetone. Many of these compounds can be easily removed by contact with water or passage through alumina or other adsorbents.
Due to the requirement of extensive purification and cost associated with obtaining high purity isoprene, isoprene derived from petroleum processes have not been a viable starting material for further processing to produce fuels. Cost effective methods are desirable for converting biologically produced isoprene to valuable chemical products, fuel in particular, taking advantage of the high purity and/or the unique impurity profiles of bioisoprene compositions.
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